Recently, surface acoustic wave elements have been investigated intensively to be used as filters. Particularly, a portable communication apparatus has been developed and adapted for high frequency. Therefore, the development of a surface acoustic element, especially a surface acoustic filter, has been pursued. In the past, various methods to form a filter of high frequency band by using surface acoustic wave element, especially a filter of several 100 MHz, have been proposed. The typical examples include, a ladder-type filter composed of a plurality of surface acoustic wave resonators disclosed in Japanese Laid-Open Patent Publication No. 52-19044, a multi-electrode type filter disclosed in Japanese Laid-Open Patent Publication No. 58-154917 and a longitudinal mode type filter in which adjacent surface acoustic wave resonators are provided and a coupling between resonators is utilized. See Japanese Laid-Open Publication No. 3-222512, No. 61-230419 and No. 1-231417. In general, many characteristics of the surface acoustic wave filter are determined by the characteristics of the substrate. In particular, a pass band width of the filter and the temperature characteristic and the like are determined by the characteristics of the substrate.
Recently, due to the development of portable communication apparatus, the frequency band to be used becomes high, that is a region of quasi-micro wave. Particularly, in the U.S., "PCS" system is put to practical use, while in European countries, "PCN" system is put to practical use. In these systems, 1.8 GHz band and 1.9 GHz band are used respectively as a frequency. In a conventional portable communication system, for example, the Japanese analog system, 915 MHz to 940 MHz is used as a transmission frequency, 860 MHz to 885 MHz is used as a receiving frequency and a transmission/receiving frequency interval is 30 MHz. However, in the PCS system (1.8 GHz band), 1805 MHz to 1880 MHz is used as a receiving frequency, 1710 MHz to 1785 MHz is used as a transmission frequency and a transmission/receiving frequency interval is 20 MHz. In the PCS system, although the used frequency band becomes high as above-mentioned, the transmission/receiving frequency interval becomes narrower than that of the conventional portable communication system (30 MHz). The communication system is not a simultaneous transmission and reception, but a time-division system, however, it is required to secure a receiving band attenuation for a transmission filter and it is required to secure a transmission band attenuation for a receiving filter. Consequently, it is very difficult to obtain the filter used for PCS system. As a result, a surface acoustic wave filter composed of only one stage can not secure the required attenuation. Therefore, it is required to connect the surface acoustic wave filter with a dielectric filter in series to secure the sufficient attenuation. However, when a surface acoustic wave filter is connected with a dielectric filter in series, the insertion loss within a band at ordinary temperature is increased.
The inventors of this invention have concluded that it might be possible to decrease the insertion loss within a band at ordinary temperature to as small an amount as possible by connecting a surface acoustic filter having a predetermined frequency temperature characteristic with a dielectric filter having a predetermined frequency temperature characteristic in series.